The implementation of a low cost, low insertion loss, dense wavelength division multiplexer (DWDM) or demultiplexer (DWDD) is of great advantage in high capacity optical systems. Unfortunately, most DWDM of DWDD devices have high insertion loss or high cost associated with them.
U.S. Pat. No. 5,809,190 (Chen), issued on Sept. 15, 1998, discloses an unbalanced Mach-Zehnder (UMZ) interferometer capable of accomplishing dense wavelength division (DWD) multiplexing with low insertion loss. More particularly, Chen discloses apparatus and method of making a fused dense wavelength division multiplexer (DWDM) using a fused-biconical taper technique. The DWDM comprises multiple Multi-window Wavelength Division Multiplexers (MWDMs) which are cascaded in several stages where the MWDMs in each stage have an identical window spacing. For an N-channel DWDM, there are a predetermined plurality of DWDMs in each stage, and the stages are cascaded to form the MWDM. Unfortunately, the UMZ is by nature sensitive to temperature fluctuations of the environment, and typical temperature fluctuations expected in the terminal environment can render the DWDM device unusable. Therefore, the disclosed UMZ device is unstable because of the occurrence of variations in phase due to temperature fluctuations. A similar device is also discussed in an article titled "Fused-Coupler Technology for DWDM Applications" by F. Gonthier in the magazine Fiber Optic Product News, September 1998, at pages 54 and 56.
U.S. Pat. No. 5,647,037 (Byron), issued on Jul. 8, 1997, discloses an optical band-pass filter with tunable performance achieved by adding a variable time delay in one of two output legs of a 3 dB splitter. Both output legs containing separate matched Bragg grating reflectors at separate predetermined distances from a central optical coupling region of the filter to reduce any noise component in a channel signal by a predetermined amount. The optical filter is able to redirect the channel signal into either a launch leg or a band-pass leg of the filter. The filter, however, does not provide for any compensation for variations in phase due to environmental fluctuations.
It is desirable to provide an ultra-stable optical wavelength division multiplexing and/or demultiplexing arrangement to automatically compensate for any change in signal drift and/or transmission properties of an included filter.